Fix clippy with latest nightly

[Nashtare]

Description

Latest versions of the compiler broke clippy runs by warning about redundant imports, cf:

error: the item `String` is imported redundantly
    --> src/lib.rs:1640:17
     |
1640 |     use utils::{String, ToOwned};
     |                 ^^^^^^
     |
    ::: /Users/nashtare/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/src/rust/library/std/src/prelude/mod.rs:125:13
     |
125  |     pub use super::v1::*;
     |             --------- the item `String` is already defined here

To alleviate this, we fully exclude the use of the std prelude by marking the library as #![no_std], and then import std conditionally through extern crate std;.
Additionally, some actual redundant imports were not caught previously and have been cleaned up altogether.

[LindaGuiga]

LGTM, thanks!